1. Field of the Invention
The present invention relates to load indicator washers and in particular to an assembly for providing underload and overload indication.
2. General Background
In many types of construction, it is frequently required that joints between two separate work pieces made with fasteners such as studs or bolts meet a certain minimum load along the longitudinal axis of the fastener. This minimum axial load is required to insure that the joint will withstand normal operational stresses. Also, joints are often designed with maximum axial loads. As an alternative to torque wrenches, which may not always provide a true representation of axial load, a variety of load indicators have been utilized in the known art.
U.S. Pat. No. 4,149,446 entitled "Load Indicator Washer" discloses a load indicator washer in the form of a parallelogram including bearing surfaces on one face which extend inwardly a short distance from two opposite ends of the washer. An arcuate portion between the bearing surfaces has a conical portion extending symmetrically thereabout and a hole to receive a bolt. The arcuate and conical portions each act as springs with the relative deflection of each indicating the load applied to a fastener such as a mine roof bolt.
U.S. Pat. No. 3,978,761 entitled "Fastener Assembly" discloses a flat washer, an elastically deformable washer, and a nut all enclosed in a shroud to insure use of the components in the proper order. Drive means coupled to the nut is designed to break at a predetermined torque level.
U.S. Pat. No. 4,571,133 entitled "Loading Washer Assembly" discloses a pair of rigid washers, each having radial upstanding ribs angularly offset relative to the other washer. A normally flat ring spring positioned between the washers is deformed into a wave spring to spring load the washers.
U.S. Pat. No. 3,174,386 entitled "Indicating Washer" discloses a stress indicating washer having a compressive column designed to collapse upon application of a predetermined axial stress when used in conjunction with a fastener such as an expansion bolt.
U.S. Pat. No. 3,476,009 entitled "Spring Washer" discloses an elastic load supporting washer of generally frustoconical shape having inner and outer surfaces of undulated configuration.
U.S. Pat. No. 4,006,661 entitled "Reusable Torque Limiting Fastening Device" discloses a torque nut with a recessed area which receives concave washers providing the necessary resistance to produce the desired bolt tensile load when the outer face of the nut contacts the work surface.
U.S. Pat. No. 4,020,734 entitled "Tension Indicator For Fastener Means" discloses a pair of washers having concave central portions which face in opposite directions in the assembled condition. Outwardly extending fingers on each washer interlock with the opposed washer and a specific compression between washers indicates that the desired load is achieved.
U.S. Pat. No. 4,029,379 entitled "Electrical Bus Duct With Torque Indication Nut" discloses a nut with an engaging section, a driving section and a bridging section which fractures upon reaching a predetermined value at the driving section.
U.S. Pat. No. 4,072,081 entitled "Tension Indicating Washer Unit" discloses a preassembled unit incorporating an upper washer, a lower conical washer, and a ring gauge clamped between the washers to prevent movement upon compression to a predetermined amount.
U.S. Pat. No. 4,205,572 entitled "Saw Blade Retainer And Kickback Clutch Assembly" discloses a blade clamping washer, spring washer, and blade screw formed as a permanent assembly in a retainer cup. Circular markings on the outer face of the clamping washer and spring washer provide a visual indication of proper tensioning.
U.S. Pat. No. 4,504,180 entitled "Multi-Headed Screw" discloses a screw having a thin neck portion designed to be sheared at a predetermined tightening torque.
The publication, NASA Tech Briefs, Winter 1982, discloses at page 192 a built-in gauge wherein a screw is attached through a bolt by a jamnut and a cruciform washer is positioned between the head of the bolt and the head of the screw. When the bolt is loaded with a minimum stress, the preset gap is taken up and the screw head clamps the cruciform washer and prevents it from rotating. This provides an indication that a minimum load has been reached but does not provide an indication if the maximum load is exceeded.
The known art provides a variety of washers aimed at supporting loads or indicating when a certain load is achieved but does not provide an assembly capable of indicating when a certain load is achieved in addition to providing overload and underload indications.